Over the last couple of days I have been seeing posts on different sites including Lifehacker and PC World about how Windows Vista SP2 has freed up large amounts of hard drive space after install. As is typical these posts are a result of people not doing their homework and just running with the most sensational thing they can find.

I decided to do only a very quick and basic investigation into these reports with my own Windows Vista install. The base size of a Windows Vista SP0 install is 8 GB. After Service Pack 1 is installed the size increases to 9.4 GB. Then after Windows Service Pack 2 is installed the size increases (yet again) to 10.2 GB.

So after seeing just the baseline numbers and realizing that it is not decreasing the used space by magic, what exactly is happening? Over the lifetime of an average Windows Vista install (or any other OS) there are updates that get downloaded and installed. Typically these installs are around a few megabytes but they make backup copies of themselves to the computer. These are typically in the form of Uninstall folders so that if there is a conflict or problem it can be removed to resolve the issue.

With Windows XP it was fairly common to find that an update caused such a severe failure it could even prevent a computer from booting into Normal Mode. By uninstalling the offending update it would resolve the issue. Sometimes it would be the cause of a failed install or even a driver conflict but the point is that these uninstall files could be a life saver.

Service Packs are compilations of individual updates to an operating system (SP2 includes 600 of them). Most of these individual updates eventually get superseded by other updates. This creates a lot of duplicates that are almost always unneeded. Simply deleting these duplicates can frequently free up massive amounts of space, but is not recommended.

Then comes the Windows Side-By-Side (winsxs) folder which is more than bloated. On my test Windows Vista, the post SP2 size of the winsxs is 6.68 GB. This might sound like a lot but in reality this is very small and will grow rapidly with heavy computer usage. Every time you install a program, driver, update, etc., files are added to the winsxs directory as a sort of backup.

An interesting test to perform (on a test machine of course) is to navigate to the Media Player folder in the Program Files directory. Delete a major file from this directory (might need to do this via a secondary OS), for example the actual wmplayer.exe. Then go to your Start Menu and attempt to launch Media Player. AutoMagically it will launch and operate normally! This is possible because the operating system realizes that it is missing and then defaults over to the executable in the winsxs. Duplicates can be a major blessing especially when there is a severe file corruption or other issue preventing normal operation.

Many sites believe that the recovered space is due to compcln.exe. Basically it is a tool that cleans up after itself and removes duplicates and unneeded files. The problem with this theory is that according to the Microsoft Technet website it is only for removing the aforementioned uninstall files for Windows Vista Service Pack 2. This is a utility designed for technicians to create deployable images to their users.

I admit that I do not know the exact reason that this is being claimed by many users. It could be something as simple as emptying temporary files or deleting restore points. Until Microsoft chimes in on this we will need to wait for someone with more time on their hands than I have to research the exact reasons that some users are experiencing the Free Space Phenomenon.